This invention relates to a timer circuit for use in protective relaying applications, and more particularly to such a circuit suitable for use in three phase a-c power system applications.
As a result of advancements in solid state technology, static protective relays have been developed. Exemplary circuits for phase comparator type static relays are disclosed in U.S. Pat. No. 4,034,269, entitled, "Protective Relay Circuit", issued July 5, 1977, this patent hereby incorporated by reference in the present application. In the phase comparator method of measurement, the relay operates by measuring the angle between two or more phasors derived within the relay. For example, as discussed more fully in U.S. Pat. No. 4,034,269, (see FIG. 6), in one form of such phase comparator method of operation, phase coincidence circuitry provides an output signal when phase coincidence exists between a polarizing signal, V.sub.AB, and an operating signal, I.sub.AB Z.sub.R - V.sub.AB. An integrating timer receives the phase coincidence output signal and produces a timer output signal which operates a trip coil when phase coincidence occurs for a predetermined minimum time. Within the intended reach of the relay, for 60 hertz applications, such phase coincidence is greater than 1/4 cycle (4.167 ms). Hence, the operate time, or pickup time, of the timer is ordinarily set to operate on an input block width of 4.167 ms. The timer also includes circuitry for providing a dropout time delay to reset following the termination of one such operate time. Such timers are generally diagrammatically designated to include both pickup and dropout information. For example, in the designation, 4/5, the top number, 4, would represent the time delay pickup in milliseconds, while the bottom number, 5, is the drop-out time delay, also in milliseconds. In this case, if a continuous phase coincidence signal of 4ms or more is present, a timer output signal is produced. When the phase coincidence signal is removed, the timer output signal continues for the dropout time of 5 ms. If no phase coincidence input signal is present for more than 4 ms, no timer output signal is produced. It is to be noted that the number 4 is for convenience; it actually represents the number 4.167 ms.
Presently available timer circuits for protective relay applications generally include an RC circuit for providing the necessary time delay for pickup and/or dropout. For example, one such relay timer circuit is disclosed in column 8, line 49, through column 10, line 20 and shown in FIG. 5 of the previously referenced U.S. Pat. No. 4,034,269. Further information on such relay timer circuits can be found in, "Printed Circuit Cards for MOD III Static Relay Equipment for Use in Static Line Relays", Instructions GEK-34158A, available from Power Systems Management Department, General Electric Company, Philadelphia, Pa.
In such an RC type timer circuit, the timing process is necessarily accomplished through ramping voltage signals which ramp up and down in an exponential manner. This means that, if the phase coincidence signal to such a timer has a short time duration "gap" between input blocks, such a timer is likely to be rapidly reset through its exponentially ramping voltage signal. As a result, such a timer is not responsive to the input block before the "gap". It is to be noted that such short time duration "gaps", or "holes", are expected to occur in many relay applications where phase comparator operation is employed. Accordingly, it would be desirable if the timer circuit were responsive to the blocks before and after such time "gaps". That is, it would be desirable if, in pickup operation, the timer circuit had the capability to integrate the blocks before and after such time "gaps". Another difficulty with presently available timer circuits is found in relay applications involving three phase a-c power systems. In such three phase power system applications, it is normally necessary to provide a separate timer circuit for each phase. Further, it is also generally necessary to provide timer adjustment circuitry for each of the separate three phase timers. For example, three variable resistors may be provided for each phase of the three phase system to provide adjustment of the following: transient pickup time, steady state pickup time, and dropout time. This would require the use of nine such variable resistors. Cost considerations and space limitations make such a circuit undesirable. Further, nine such variable resistors require considerable testing and setting time.
Accordingly, it is a general object of this invention to provide a timer circuit for a protective relay circuit in which the timer includes ramping circuit means for producing a substantially linear voltage output signal.
It is another object of this invention to provide such a timer circuit in which, if the timer has a short time duration "gap" in its phase coincidence signal, the timer is only partially reset so as to be responsive to the effect of the blocks before and after such "gaps".
It is another object of this invention to provide such a timer circuit for a three phase power system in which some circuitry is shared between all three phases to respectively permit single simultaneous adjustment for all three phases of one or more of the following: transient pickup time; steady state pickup time; and dropout time.